Recently, various types of biosensors using a specific catalysis function of an enzyme have been developed as a quantitative analysis method for a saccharide such as sucrose or glucose.
Now, a quantitative analysis method for glucose will be described as an example of the quantitative analysis method for a saccharide included in a sample. As an electrochemical quantitative analysis method for glucose, a method using an enzyme of glucose oxidase (EC1.1.3.4; hereinafter abbreviated as GOD) and an oxygen electrode or a hydrogen peroxide electrode is generally known.
GOD selectively oxidizes a substrate of β-D-glucose into D-glucono-δ-lactone by using oxygen as an electron mediator. In the oxidation reaction caused by GOD in the presence of oxygen, oxygen is reduced to hydrogen peroxide. The amount of thus reduced oxygen is measured with the oxygen electrode or the amount of thus increased hydrogen peroxide is measured with the hydrogen peroxide electrode. The amount of reduced oxygen or the amount of increased hydrogen peroxide is proportional to the content of glucose in a sample, and therefore, the glucose is measured on the basis of the amount of reduced oxygen or the amount of increased hydrogen peroxide.
In the aforementioned method, the glucose included in the sample can be accurately measured by utilizing the specificity of the enzyme reaction. However, as is presumed from the reaction process, this method has a disadvantage that it is largely affected by the concentration of oxygen included in the sample, and when the sample includes no oxygen, the measurement cannot be performed.
Therefore, a glucose measuring biosensor using, as an electron mediator, not oxygen but an organic compound or a metal complex such as potassium ferricyanide, a ferrocene derivative or a xenon derivative has been developed. In this glucose measuring biosensor, a reductant of the electron mediator resulting from an enzyme reaction is oxidized on a working electrode, thereby obtaining the concentration of glucose included in a sample on the basis of an oxidation current flowing in the oxidation. At this point, a reaction in which an oxidant of the electron mediator is reduced so as to generate the reductant of the electron mediator is proceeded on a counter electrode. When an organic compound or a metal complex is thus used as an electron mediator instead of oxygen, a reagent section can be formed by stably and accurately holding a known amount of GOD and the electron mediator on the electrode, and therefore, glucose can be accurately measured without being affected by the concentration of oxygen included in the sample. Also, since the reagent section including the enzyme and the electron mediator can be integrated with an electrode system in an almost dry state in this case, a disposable glucose measuring biosensor based on this technique is recently regarded remarkable. A typical example of such a biosensor is a biosensor described in Patent Document 1, Japanese Laid-Open Patent Publication No. 3-202764. In using a disposable glucose measuring biosensor, the concentration of glucose can be easily measured with a measuring apparatus by simply introducing a sample into the sensor removably connected to the measuring apparatus.
Exemplified measuring procedures for a blood glucose level (that is, a glucose concentration in blood) using the aforementioned disposable glucose measuring biosensor will be described.
First, a measurer takes a glucose measuring biosensor out of a package containing a desiccant and fits it on a measuring apparatus. Thereafter, blood obtained by, for example, tapping a finger tip or the like with a needle is applied on the glucose measuring biosensor, and after a given period of time, the blood glucose level of the measurer is displayed on a displaying section of the measuring apparatus.
Patent Document 1: Japanese Laid-Open Patent Publication No. 3-202764